public class Computer{
    public void poweron(){
        System.out.println("打开电脑");
    }
    public void poweroff(){
        System.out.println("关闭电脑");
    }



    public void useDevice(USB usb){
        usb.opendevice();

        //向下转型（需要调用子类特有方法）
        if(usb instanceof Mouse){
            Mouse mouse = (Mouse) usb;
            mouse.click();
        }else if (usb instanceof Keyboard){
            Keyboard keyboard = (Keyboard) usb;
            keyboard.input();
        }
        usb.closedevice();
    }
}
